1. Field of the Invention
This invention relates to distributed fiber sensors based on spontaneous Brillouin scattering—and more specifically to a distributed fiber sensor that uses a single-frequency laser to generate the pulsed light and a Brillouin fiber ring laser to provide the optical local oscillator (OLO).
2. Description of the Related Art
The application of optical fiber technology to the problem of distributed sensing offers the potential ability to determine a physical parameter such as temperature or strain as a function of the position along an optical fiber cable over lengths up to tens of kilometers. Typically, the measurements are performed using optical time-domain reflectometry (OTDR), which detects pulsed of light backscattered from the optical fiber.
Distributed fiber sensors based on Brillouin scattering have received attention since the Brillouin frequency shift in an optical fiber was found to linearly depend on the fiber strain and temperature in 1989 (T. Horiguchi, T. Kurashima, and M. Tateda, “Tensile Strain Dependence of Brillouin Frequency Shift in Silica Optical Fiber,” IEEE Photonics Technol. Lett., 1 (1989), p. 107. D. Culverhouse, F. Farahi, C. N. Pannel, D. A. Jackson, “Potential of Stimulated Brillouin Scattering as Sensing Mechanism for Distributed Temperature Sensors,” Electron. Lett., 25 (1989), p. 913.). The main advantage of Brillouin-based fiber sensors over conventional Raman-based systems is that standard telecommunication single mode fiber can be used to measure both temperature and strain simultaneously and unambiguously. Although both the Raman effect and Brillouin effect involve the nonlinear backscattering of the light propagating in an optical fiber, the intensity and frequency shift of the two backscattered components are much different.
As shown in FIG. 1, incident light 10 propagating in an optical fiber produces backscattering in the Rayleigh band 12, Brillouin band 14 and Raman band 16. The intensity of Brillouin backscattered light 14 is much stronger, at least one order of magnitude greater, than Raman scattering 16. Therefore, Brillouin-based sensors may offer much better signal-to-noise ratio and higher dynamic range. As also shown, the Brillouin lines 14 are frequency shifted from the launch wavelength by about ten gigahertz, while the Raman lines 16 are shifted in wavelength from the launch light by about 13 THz. If a 1550-nm light source and telecommunication-grade single mode fiber are used, the Brillouin lines appear at 1549.01 (anti-Stokes) and 1550.09 nm (Stokes) while the Raman lines appear at around 1450 (anti-Stokes) and 1650 nm (Stokes). Therefore, the Brillouin lines are still in the telecommunication wavelength region, and all the low-cost fiber optic components such as an EDFA that are commercially available for the telecommunication industry can be used in Brillouin-based fiber sensors. In contrast, the Raman lines are out of the most-commonly used wavelength region for telecommunication, and fiber optic components at the Raman wavelengths are not commercially available.
Since their discovery, extensive research has been done to develop both stimulated and spontaneous Brillouin-based distributed fiber sensors. However, the complexity and high cost of current implementations has prevented widespread commercialization. Spontaneous Brillouin-based sensor techniques offer the capabilities to simultaneously sense temperature and strain with only one pulsed laser source.
When a pulsed laser is launched into a fiber, spontaneous Brillouin backscattering of the pulsed laser is detected and analyzed. The pulsed laser can be either a Q-switched laser (usually with gigahertz-linewidth) or the combination of a CW DFB laser (with 1 MHz linewidth) and an amplitude modulator (AO/EO modulator). Temperature and strain can be simultaneously measured if both the Brillouin frequency shift and the Brillouin backscattering power are determined. The backscattering signal can be measured by either direct detection or coherent detection. Coherent detection offers much higher sensitivity and dynamic range than direct detection. However, since the frequency of spontaneous Brillouin backscattering is down-shifted by approximately 11 GHz (“the Brillouin frequency shift”) from that of a 1.55 μm pump light, the beat frequency (coherent self-heterodyne signal) between the spontaneous Brillouin backscattering and the pump light is extremely high and the signal lies out of the bandwidth of most commonly-used conventional heterodyne receivers, which requires expensive microwave coherent detection. Since the thermal noise power of a photo receiver is proportional to the receiver bandwidth, microwave detection suffers from low sensitivity and high noise level. To obtain an acceptable SNR (signal-to-noise ratio), the beat signal needs to be averaged over hundreds and thousands of times, which is a time-consuming process and results in measurement times of typically more than ten minutes per measurement.
There have been some efforts to optically shift the frequency of the optical local oscillator (OLO) to set the Brillouin/OLO beat frequency within the bandwidth of a conventional heterodyne receiver. Low-frequency heterodyne detection offers not only high sensitivity, but also the opportunity of real-time data processing using low-cost commercially available electronics. These efforts include the use of a mode-locked Brillouin fiber laser 28 as shown in FIG. 2 (V. Lecoeuche et al., “Brillouin Based Distributed Fiber Sensor Incorporating a Mode-Locked Brillouin Fiber Ring Laser,” Opt. Commun., 152 (1998), p. 263; V. Lecoeuche et al., “25 km Brillouin Based Single-Ended Distributed Fiber Sensor for Threshold detection of Temperature or Strain,” Opt. Commun., 168 (1999), p. 95; and V. Lecoeuche et al., “20-km Distributed Temperature Based on Spontaneous Brillouin Scattering,” IEEE Photon. Technol. Lett., 12 (2000), p. 1367) and a high-speed electro-optic (EO) phase modulator 52 as shown in FIG. 3 (H. Izumita et al., “Brillouin OTDR Employing Optical Frequency Shifter Using Side-Band Generation Technique with High-Speed LN Phase Modulator,” IEEE Photon. Technol. Lett., 8 (1996), p. 1674; and M Nikles et al., “Simple Distributed Fiber Sensor Based on Brillouin Gain Spectrum Analysis,” Opt. Lett., 21 (1996), p. 758).
As shown in FIG. 2, a spontaneous Brillouin distributed fiber sensor 24 includes a single-frequency laser 26 whose output is split into two beams by a fiber coupler 27. One beam is used to pump a mode-locked Brillouin fiber ring laser 28. The ring laser's pulsed output is sent to a sensing fiber 32 through a fiber coupler 30. The back-scattered spontaneous Brillouin scattering light is directed to optically mix with an OLO beam through two fiber couplers 30 and 36. The second beam from the single-frequency laser 26 is used as the OLO. After going through a polarization scrambler 34, the OLO is mixed with the back-scattered signal. The coherent beat signal is detected with a photo detector 38 and analyzed by heterodyne detection electronics 40.
The mode-locking behavior of mode-locked Brillouin fiber ring laser 28 originates from highly unstable periodic intensity modulation of both pump laser and the Brillouin laser. Since the Brillouin gain bandwidth in fiber is on the order of tens MHz, the cavity length of a mode-locked Brillouin laser needs to be hundreds to thousands meters so that multiple longitudinal modes can oscillate within the gain bandwidth at the same time. Thus, it is practically impossible to prevent the pump laser field circulating inside the Brillouin laser cavity from intensity fluctuation because of the finite linewidth or phase noise of the pump laser. This is true even when there is an active stabilization of the Brillouin cavity length. As a result, the mode-locked Brillouin fiber laser also suffers from intensity fluctuation.
As shown in FIG. 3, a spontaneous Brillouin distributed fiber sensor 42 includes a single-frequency laser 44 whose output is split into two beams by a fiber coupler 46. One beam is phase modulated by an EO modulator 52 and its driver 50 to generate microwave side-bands. The EO modulator's phase-modulated output is amplitude-modulated by another AO/EO modulator 56 and its driver 54. After boosting the optical power by an Er-doped fiber amplifier 57, the beam is sent to a sensing fiber 58 through a fiber coupler 60. The back-scattered spontaneous Brillouin scattering light is sent to optically mix with an OLO beam through two fiber couplers 66 and 62. A second beam from the single-frequency laser 44 is used as the OLO. The OLO is mixed with the back-scattered signal. The coherent beat signal is detected with a photo detector 64 and analyzed by heterodyne detection electronics 66.
The use of a high-speed EO phase modulator provides a very simple alternative to generate a frequency-shifted OLO. However, there are several fundamental issues associated with this approach. First, since one of the side-band frequency components is used as the OLO in this approach, the performances (such as frequency stability and repeatability, phase noise, conversion efficiency) of the OLO have dominant influence on measurement accuracy and sensitivity, and dynamic range in coherent detection. Thus, the OLO performances are dramatically dependent on performances of a microwave generator 50 or synthesizer that drives the phase modulator 52. In practice, it is extremely difficult to obtain microwave drive signals with excellent performance. Second, the high-order side-band components make the signal analysis more complex because of signal interference between the high-order components and Rayleigh/Brillouin backscattering. Finally, conventional EO modulators allow only limited modulation depths (or limited conversion efficiency from the carrier to sidebands); optically carried signals decompose over low-level sidebands and a strong optical carrier, which can easily saturate a detector in coherent detection.
Although spontaneous Brillouin-based sensors have shown considerable potential advantages over conventional Raman-bases sensors, the barriers in the current implementations to a low cost system capable of real time sensing continue to inhibit commercialization.